898 THE VASCULAR SYSTEM. 
the common stem now gives off the right subclavian artery, and then continues as a single vessel 
for some distance before it divides into the two common carotids, of which the left crosses in 
front of the trachea. This arrangement is common in many quadrumana and in some other 
manunals. 
It is only very occasionally when the number of branches from the arch of the aorta is 
reduced to two, that these consist of a right subclavian artery and of a single stem common to 
the two carotids and the left subclavian arteries. In such cases, however, the right common 
carotid crosses in front of the trachea, and the variation is one of practic ‘al importance. It does 
not appear to exist as a normal condition in any mammal. Probably it is due to fusion of the 
ventral roots of the fourth aortic arches, with absorption of the left fourth arch and the left sub- 
clavian into the stem so formed, whilst the right subclavian is relatively displaced. The two 
common carotids may arise by a common stem, and the left subclavian arise separately from 
the arch of the aorta, whilst the right subclavian springs from the descending aorta. This 
arrangement probably results from the disappearance of the fourth right arch and the fusion of 
the ventral roots of the fourth arches of opposite sides. 
Sometimes two innominate arteries, right and left, replace the three usual branches of the 
arch of the aorta. This is the normal arrangement in bats, moles, and hedgehogs. It is 
obviously the result of the disappearance of that portion of the arch which intervenes between 
the left carotid and left subclavian arteries, and the consequent fusion of these two vessels. 
In a similar way may be explained the rarer condition in which the three ordinary 
branches of the arch arise by one single stem, which divides into right and left innominate: 
arteries. In most ruminants, in the horse and in the tapir, this arrangement is constant. 
It will be evident that other combinations and modifications may be met with in the branches 
of the arch of the aorta as the result of fusions and absorption. 
The bronchial arteries obviously correspond to splanchnic segmental arteries and their 
continuations to diverticula from the walls of the gut, therefore the usual origin of the 
right bronchial artery from the first right aortic intercostal artery must result. from the 
persistence of an anastomosis between a splanchnic segmental artery and the first part of a 
somatic segmental artery ; the origin of the right from the upper left bronchial artery, which 
sometimes occurs, is due to the fusion of the roots of two splanchnic segmental arteries. The 
occasional origin of a bronchial vessel from an internal mammary artery can only result from 
the persistence and enlargement of an anastomosis between a splanchnic segmental artery and 
the ventral branch of a somatic segmental artery. The origin of a bronchial branch from a. 
subelavian artery may have the same or a different significance on opposite sides of the body. 
A bronchial artery arising from the left subclavian artery corresponds with the origin of the 
right bronchial artery from the first aortic intercostal artery ; it is due to the persistence of am 
anastomosis between a splanchnic segmental artery and the root of a somatic segmental artery, 
and the origin of a bronchial artery from a right subclavian artery may be due to a similar cause. 
It may, on “the other hand, be due to the enlargement of an anastomosis between a splanchnic 
branch of the descending ‘aorta and a splanchnic branch of the fourth right aortic arch. When, 
as occasionally happens, the bronchial artery arises from the inferior thyroid, it is due to the 
persistence and enlargement of an anastomosis between splanchnic arteries. 
Intercostal Arteries.—Variations of the intercostal arteries are not very common, but: 
they are significant and interesting. Corresponding vessels of opposite sides may arise from a, 
common stem which has been formed by the fusion of the roots of two somatic segmental arteries: 
after or simultaneously with the fusion of the primitive dorsal aorte. The number of intercostal. 
arteries may be reduced, one artery supplying two or more intercostal spaces ; in these cases the 
roots of origin of some of the somatic segmental arteries in the dorsal region have disappeared, 
and the precostal anastomoses between their ventral branches have persisted. 
Occasionally the number of the aortic intercostal arteries is increased, an additional artery 
being given to the second intercostal space, which is usually supplied by the superior intercostal 
artery ; ; this is brought about by the persistence of the root of the tenth somatic segmental artery 
and the disappearance of the precostal anastomosis between the ventral branches of the ninth and 
tenth somatic segmental arteries. | Very rarely the first aortic intercostal artery sends a branch 
upwards between the necks of the ribs and the transverse processes of the upper dorsal region ; 
this branch supphes the upper intercostal spaces, the superior intercostal artery being small or 
absent, and it terminates by becoming the profunda cervicis artery. It is due to the persistence 
of the postcostal anastomoses in the upper dorsal region, and is a repetition of a condition regularly 
present in some carnivores. 
There are no very important variations of the oesophageal, pericardial, and mediastinal 
arteries. : 
Lumbar Arteries.—Variations of the lumbar arteries are very similar to those of the 
intercostal arteries, and they are due to similar causes. The lumbar arteries of opposite sides 
may arise by common stems from the back of the aorta; and the last pair of lumbar arteries 
may arise In common with the middle sacral artery. Further, a lumbar artery may have its 
area of distribution increased to the adjacent segment. 
The inferior phrenic arteries are very variable ; they may arise by a common trunk either 
from the eceliac axis or from the aorta; they may arise separately either from the aorta or from 
the coeliac axis, and more commonly from the latter vessel ; or again, one may spring from the 
aorta or ceeliac axis, and the other from the coronary, renal, or even from the superior mesenteric 
artery. 
The middle sacral artery usually springs from the back of the aorta above its bifurcation ; 
